1. Field of the Invention
The present invention relates to a liquid crystal composition and a liquid crystal display device using the same.
2. Description of the Related Art
Liquid crystal display devices using a chiral nematic liquid crystal composition containing a chiral material added into a nematic liquid crystal that forms a cholesteric phase at room temperature have been known. In such a liquid crystal display device, a chiral nematic liquid crystal composition is normally put and held between a pair of substrates having a transparent electrode, and the liquid crystal is displayed, by switching between the planer (PL) state and the focal conic (FC) or homeotropic (homeo) state by application of a pulse voltage (driving voltage) between the electrodes. In particular, a light having a particular peak wavelength is reflected selectively in the PL state.
A method of preparing a display device by adding a monomer and a polymerization initiator to a cholesteric liquid crystal composition and then polymerizing the monomer, for example, by irradiation of ultraviolet light (UV) was proposed (Nonpatent Literature 1) for monochromic displaying and expansion of view angle. However, the method caused a problem that the unreacted monomer remaining therein reacts gradually by ultraviolet light and resulted in deterioration in display performance during use of the element, because it was not possible to advance polymerization until there is no unreacted monomer any more even if the polymerization was carried out sufficiently.
In addition, methods of adding a gelling agent to a cholesteric liquid crystal composition for display superior in brightness and contrast were also known (Patent Documents 1 and 2). The methods would be expected to solve the problem of deterioration in display performance with time. However, in the liquid crystal display devices containing a gelling agent in cholesteric liquid crystal composition so far proposed, it was difficult to satisfy both whiteness and sufficient brightness during white display. For improvement in the whiteness during white display, it was necessary to adjust the helical pitch of cholesteric liquid crystal so that the selective reflection wavelength falls in the infrared range, and thus, in such a case, it was not possible to obtain a sufficiently bright display. On the contrary, if the helical pitch is so adjusted that the selective reflection wavelength of the cholesteric liquid crystal falls in the visible region, it is difficult to obtain favorable whiteness.                [Nonpatent Literature 1]: R. Q. Ma, et al., “SID 97 DIGEST”, pp. 101 to 104        [Patent Document 1]: Japanese Patent Application Laid-Open No. 2004-226552        [Patent Document 2]: Japanese Patent Application Laid-Open No. 2005-91738        